Process for the manufacture of objects with small complex cross-sections

ABSTRACT

A need often arises for micro-optical components, such as optical fibres and couplers, and micromechanical components with complex cross-sections which are different at opposite ends or, for directional couplers, at points along the length of the couplers. In the present invention, a soluble material 38 is added to at least one primary preform 35 to make a secondary preform 37 of circular cross-section. The secondary preform is then drawn to reduce the cross-section of the primary preform as required but the cross-sectional shape of the secondary preform is preserved. The soluble material is then removed and then part of the resulting product is heated and plastically deformed to give the required different ends 39 and 40 or other different cross-sections. In another aspect of the invention two soluble materials are used with different solubilities. After the most soluble material has been removed, an operation involving the less soluble material can be carried out, and then the less soluble material is at least partially removed.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to methods for the manufacture of objectswith small complex cross-sections, particularly optical fibres,micro-optical components, such as couplers, and micromechanicalcomponents.

2. Description of the Related Art

Fibre optics and integrated optic components are used in opticalinformation technology and measuring techniques. Such components employoptical waveguides which determine their properties either as a resultof their overall structure or by virtue of the cross-section of thewaveguide or of external conditions.

Optical components in which the overall structure determines propertiesinclude star junctions and directional couplers in which two or morewaveguides gradually converge or separate or are in close proximity overa defined length to such an extent that their fields interact. (SeeUnger, Optische Nachrichtentecknik, Bd.II Huthig-Verlag 1984.)

Components in which the cross-section of the waveguide determines itsproperties include integrated-optic acousto-optical beam deflectors inwhich light conducted by an elongated rectangular section of waveguideinteracts with an acoustic wave propagating along the surface of thewaveguide. (See F. Auracher, Planar Electro-optic and Acousto-opticBragg Deflectors in: Integrated Optics, Hrsg. S. Martinelli, A. N.Chester, NATO ASI Series, Plenum Publication Corp., NY 1981.)

A known problem which arises with different components of the abovetypes in optical measuring and information systems is the difficulty incoupling optical waveguides together in a low loss configuration. Inmost cases this problem can be overcome by using glass fibres which arehighly flexible and can be adapted to most dimensional conditions.However such optical fibres usually have circular cross-sections and inthe case of direct butt joints to different cross-sections and sizes ofwaveguide in various components high coupling losses occur (see A.Mahapatra et al. "Thermal Tapering of Ion Exchanged Channel Guides inGlass", Optical Letters, Volume 13, No. 2, 1988). Suitable couplingcomponents must not only match in cross-section, shape and size but mustalso allow optical fields to match; that is although a change in opticalmodes may be allowed or required in passing through the couplingcomponents, significant loss of optical energy must be prevented. Modeswhich radiate from the coupling components must not be generated.

Known solutions in the form of glass fibre bundles exist for thesecoupling problems when joining multimode waveguides having differentcross-sections (see H. Naumann, G. Schroder, Bauelemente der Optik,Hanser-Verlag) or stacked optical waveguides--see U.S. Pat. No.4,530,565 (David A. Markle). All these solutions are based on couplersformed by subordinate waveguides having cross-sections which are broughttogether at the ends of the coupler to conform with the differentcross-sections of the waveguides which are to be coupled. The abovementioned U.S. Pat. No. 4,530,565 describes such an arrangement forjoining square waveguides to elongated cross-sections or large radiusarcuate cross-sections.

However no process has been disclosed for manufacturing couplingelements according to the above principles where waveguides havecross-sections small enough to allow mono-mode operation in at least onedimension. Such elements are used, typically, in integrated opticcomponents for measurement and information technology.

As described in U.S. Pat. No. 3,989,495 and U.K. Patent Application2,189,480, optical components having complex cross-sectional shapes canbe drawn by plastic deformation from preforms including additional glasswhich is etched away after drawing.

In the field of endoscopes for medical and technical diagnostic uses,where bundles of optical fibres fixed to each other at each end butunattached throughout the rest of their length are used, methods ofmanufacturing are known in which individual fibres are surrounded with asoluble glass before bundling and the resulting preform is then drawn togive the required cross-section (see such U.S. Pat. Nos. 4,389,089,3,669,772 and 3,830,667).

SUMMARY OF THE INVENTION

According to a first aspect of the present invention there is provided amethod of manufacturing objects with very small transverse dimensionsfrom material which is capable of being drawn plastically, comprisingthe steps of

forming a secondary preform from at least one primary preform formed ina first material by combining second material with the primary preform,the first and second materials both being capable of plastic deformationand one of the said materials being capable of removal from the othermaterial using a removal agent,

reducing the viscosity of a zone of the secondary preform whileplastically drawing out the secondary preform to give an elongatedproduct with a reduced cross-section, the secondary preform having across-sectional boundary whose shape, as defined by relative dimensions,is preserved during drawing by the presence of the first and/or secondmaterials, and

removing the said one material from at least part of the said elongatedproduct using the said agent,

characterised in that the method includes

plastically deforming the material of the product, after the said onematerial has been at least partially removed, to reshape the productpermanently.

Usually, the said one material is the second material and the primarypreform or preforms have a cross-sectional outer or inner peripherywhich is preserved during drawing by the presence of the secondmaterial.

The cross-sectional shape of the secondary preform may be circular orsquare but with a square cross-section the diagonal dimension of thesquare must be large compared with the largest cross-sectional dimensionof the primary preform. For example the diagonal should be at leasttwice as long as the said largest cross-sectional dimensions. Secondarypreforms with other cross-sectional shapes may also be used providedthey are geometrically regular with respect to an axis normal to thecross-section and the largest cross-sectional dimension is largecompared with the largest cross-sectional dimension of the primarypreform.

For optical components, such as optical fibres and couplers, and someother components the first and second materials are vitreous materials.The removal agent is usually a liquid or gas which dissolves the secondmaterial physically or chemically.

Complex cross-sections can be etched from drawn secondary preforms bytaking advantage of the high etching ratios which are possible betweencertain glasses and etchants. Etching ratios of between 100/1 and 1000/1or more are possible. For example glasses of types LaKN12 and SSK50,both of which are available from the Schott glassworks, are suitable,the former being much more difficult to etch than the latter.

An advantage of the first aspect of the invention is that parts of drawnprismatic fibres forming the elongated product can be manipulated tomake optical coupling elements for devices with widely differentcross-sectional shapes. Several different kinds of coupling elements anddirectional couplers can be made in this way.

The objects of the present invention include the provision of amanufacturing process for couplers which have the small dimensions formono-mode operation mentioned above.

Making non-prismatic optical elements by manipulating prismatic fibresas outlined above, allows another object of the invention to beachieved, that is the production of coupling elements and directionalcouplers in which waveguides, or bundles, or other arrangements ofwaveguides are used which, at one end, are mono-mode with respect to onedimension and therefore suitable for coupling to a strip-waveguideoptical fibre, while at the other end the cross-section is suitable forforming a high efficiency coupling with a circular or rectangularmulti-mode optical waveguide. However a problem which may arise duringmanipulation is that the drawn product may be distorted in an unwantedway.

According to a second aspect of the present invention there is provideda method of manufacturing objects with very small transverse dimensionsfrom material which is capable of being drawn plastically, comprisingthe steps of

forming a secondary preform from at least one primary preform formed ina first material by combining second material with the primary preform,the first and second materials both being capable of plasticdeformation, and one of the said materials being capable of removal fromthe other material using a removal agent,

reducing the viscosity of a zone of the secondary preform whileplastically drawing out the secondary preform to give an elongatedproduct with a reduced cross-section, the secondary preform having across-sectional boundary whose shape, as defined by relative dimensions,is preserved during drawing by the presence of the first and/or secondmaterials, and

removing the said one material from at least part of the said elongatedproduct using the said agent,

characterised in that

the first material includes third and fourth materials, the thirdmaterial being removable from the fourth material using a furtherremoval agent, and

the method includes at least partially removing the third material fromthe fourth material after the said one material has been at leastpartially removed from the first material.

The third material may be used to grip the drawn first material, afterthe second material has been removed, to allow the first material to bemanipulated so helping to overcome the problem of unwanted distortionduring manipulation mentioned above. The third material may then beentirely removed.

According to a third aspect of the present invention there is provided amethod of manufacturing objects with very small transverse dimensionsfrom material which is capable of being drawn plastically, comprisingthe steps of

forming a secondary preform from at least one primary preform formed ina first material by combining second material with the primary preform,the first and second materials both being capable of plasticdeformation, and one of the said materials being capable of removal fromthe other material using a removal agent,

reducing the viscosity of a zone of the secondary preform whileplastically drawing out the secondary preform to give an elongatedproduct with a reduced cross-section, the secondary preform having across-sectional boundary whose shape, as defined by relative dimensions,is preserved during drawing by the presence of the first and/or secondmaterials, and

removing the said one material from at least part of the said elongatedproduct using the said agent,

characterised in that

the said product includes a portion at one end which has not been drawn,or has been only partially drawn.

The third aspect of the invention can be used in making endoscopes wherea bundle of optical fibres may form the primary preform or the primarypreform may include at least two such bundles, one to conduct light toilluminate an object, and one to receive light for an image of theobject. The end of the bundle which has not been drawn, or onlypartially then gives an enlarged image.

For optical components made according to all these aspects, it is nearlyalways necessary to coat the product after the, at least partial,removal of the second material in order to set the numerical aperture ofthe component by choice of the coating material. The coating is alsouseful in facilitating polishing and locating the component in its finalposition.

BRIEF DESCRIPTION OF THE DRAWINGS

Certain embodiments of the invention will now be described withreference to the accompanying drawings, in which:

FIG. 1(a) illustrates a method used in the invention in which a preformis drawn plastically,

FIGS. 1(b) and 1(c) show cross-sections before and after drawing,respectively,

FIG. 2(a) shows a preform for use in making certain optical fibres andcouplers,

FIG. 2(b) shows an intermediate stage in a method embodiment accordingto the first aspect of the invention,

FIGS. 2(c) and 2(d) show optical couplers made from the preform of FIG.2(a).

FIGS. 3(a) and 3(b) show cross-sections of a preform for an opticalcoupler for connecting an array of optical devices to a single fibre,and a coupler made from the preform, respectively,

FIGS. 4(a) and 4(b) show the cross-section of a preform and aperspective view of a coupler made from the preform, respectively, thecoupler being suitable for connecting a rectangular cross-sectionoptical fibre to a device requiring an elongated rectangularcross-section for coupling.

FIGS. 5(a) and 5(b) show the cross-section of a preform and aperspective view of a coupler made from the preform, respectively, thecoupler being suitable for connecting a circular cross-section opticalfibre to a device requiring an elongated rectangular cross-section forcoupling,

FIG. 6(a) shows a cross-section of a preform for polarisation-preservingcouplers, in which material which is later removed in a central regionseparates optical waveguides,

FIG. 6(b) is a perspective view of a coupler made from the preform ofFIG. 6(a),

FIG. 7(a) shows a cross-section of a preform for anotherpolarisation-preserving coupler, in which two optical waveguides areadjacent to one another (but later separated except in a centralregion),

FIG. 7(b) is a perspective view of a coupler made from the preform ofFIG. 7(a),

FIG. 8(a) shows a cross-section of a preform for anotherpolarisation-preserving coupler in which positive location between twooptical waveguides is provided,

FIG. 8(b) is a perspective view of a coupler which can be made eitherfrom the preform of FIG. 8(a) or from two separate optical fibres with alongitudinal ridge and a longitudinal groove, respectively,

FIGS. 9(a) and 9(b) show the cross-section of a preform and aperspective view of a coupler made using the preform, respectively, thecoupler including a coating which interacts with light,

FIGS. 10(a) and 10(b) show the cross-section of a preform and aperspective view of a sensor or light emitter made from the preform,respectively,

FIGS. 11(a) to 11(d) show different types of optical fibres which can bebundled together in making an endoscope,

FIGS. 12(a) and 12(b) show the cross-section of a preform, and anenlarged portion of the cross-section respectively, for use in making anendoscope,

FIG. 13 is a perspective view of bundled optical fibres for an endoscopewith one end which has not been drawn,

FIGS. 14(a) and 14(b) show the cross-section of a preform for making anoptical coupler and illustrate an operation of manipulating a productdrawn from the preform after a first type of soluble glass has beenremoved and using another type of soluble glass, which is later removed,as an aid in carrying out the manipulation,

FIG. 15(a) shows the cross-section of a preform employing two differentsolubility glasses,

FIG. 15(b) is a perspective view of a product drawn from the preform ofFIG. 15(a) being joined to a diode array before the second glass isremoved,

FIG. 15(c) is a perspective view of the final product and diode arraywith the second glass partially removed, and

FIGS. 16(a) and 16(b) show the cross-section of a preform and aperspective view of a radio-isotope separator made from the preform,respectively.

DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EXEMPLARY EMBODIMENTS

A method of manufacturing virtually prismatic optical waveguides havingcomplex cross-sections is first explained as a preliminary to describingembodiments of the invention. In FIG. 1(a), a primary preform 10 of acidinsoluble glass and having a complex cross-section is formed into asecondary preform 11 having a circular cross-section by the addition ofacid soluble glass 12. FIG. (1b) shows the cross-section of thesecondary preform as it appears at a point 13 in FIG. 1(a).

The object is to produce a glass fibre 10' (FIG. 1(c)) which has thesame shape, that is the same relative dimensions, as the preform 10 butis much smaller in cross-section. The method consists of heating theglass, for example by means of a resistance furnace or hydrogen burnersin a zone 14 as indicated by the groups of arrows 15. Heating the glassreduces its viscosity in the zone and at the same time a tensilelongitudinal load is applied to the preform in the direction of thearrow 16 to draw a fibre 11' from the secondary preform by plasticdeformation. The shape of the preform 10 is preserved in the fibre 11'as shown in cross-section in FIG. 1(c).

The drawn fibre is then usually cut off in the usual way to the left (asshown in FIG. 1(a)) of the zone 14 although in some objects madeaccording to the invention at least part of the preform is required.

The last step in the method is to dissolve or leach the acid solubleglass 12 from the fibre 11' to produce a fibre having the samecross-sectional shape as the preform 10 but of very much reduceddimensions.

Any of the well known soluble and insoluble glass may be used and inthis context the term insoluble means relatively insoluble compared withthe soluble glass, or insoluble by an agent which dissolves the solubleglass.

FIG. 2(a) shows a secondary preform 20 which can be used to manufacturea number of different types of coupling elements having different endconfigurations. A stack of rectangular section glass fibres 21, in thisexample four such fibres, is contained in supplementary material 22which is a glass of comparatively high chemical or physical solubilityrelative to the fibres 21. The secondary preform 20 has a circularcross-section by virtue of the addition of the relatively highsolubility glass 22. The secondary preform 20 is drawn in the wayillustrated in FIG. 1(a) and then after cutting the drawn product fromthe preform, the supplementary material 22 is dissolved leaving, in thisexample, four optical waveguides 21' of greatly reduced dimensions butthe same shape as the preform optical waveguides of FIG. 2(a). Thewaveguides 21' may be cemented or fused together to form different typesof coupling elements. For example in FIG. 2(c) the waveguides 21' arefixed together at one end 23 but are heated and shaped by plasticdeformation to be spaced apart at the other end 24. Such a coupler maybe used to couple several waveguides at the end 24 to a single waveguidehaving a larger cross-section at the end 23. In another arrangementshown in FIG. 2(d) the fibres 21' are heated, shaped and joined side byside at one end 25 and above one another at the other end 26 to providea coupling component which is suitable for coupling a square sectionwaveguide at the end 26 with a wide flat strip waveguide at the end 25.In a variation of this process the soluble material 22 is not initiallyremoved at the end 26 so that the waveguides 21' are held together whilein the intermediate part of the coupler these waveguides are shaped andbrought together to form at the end 25 the required elongated shape. Thematerial 22 is then removed at the end 26 where the waveguides 21' arefused together and then the whole coupler is given a suitable coating.

A secondary preform 28 for an optical coupler for coupling individualoptical fibres to a multi-mode fibre is shown in FIG. 3. The secondarypreform contains a large number of rectangular-section opticalwaveguides 29 arranged in a two-dimensional array within soluble glass30. The number and layout of the optical waveguides 29 is determined bythe devices to be coupled to one end of the finished coupler. Forexample if a two-dimensional diode array is to be coupled to amulti-mode optical fibre then the spacing of the waveguides 29 dependson the layout of the diodes in the array. However, account must also betaken of the reduced dimension of the secondary preform after it hasbeen drawn, the scale factor being the ratio of the preform diameter tothe diameter of the drawn fibre. After drawing and cutting the solubleglass 30 is removed from one longitudinal part 31 but the glass 30 isretained at one end 32 where its spacing and layout correspond with thedevice to be coupled at that end, for example the diode array. At theother end 33, fibres 29' corresponding to the optical waveguides 29 areheated, shaped and fused together to provide a shape which correspondswith a multi-mode optical waveguide to be coupled at the end 33.

In variations of the arrangements shown in FIG. 3(a) and 3(b) the rowsof waveguides 29 may be provided with spacings suitable for theconnection of several linear arrays of diodes; and the material betweenthe waveguides 29 need not be formed entirely of material to be removedafter the drawing process.

An embodiment is now discussed in which a particularly complexcross-section is etched from the secondary preform. In FIG. 4a a primarypreform 35 can be regarded as a sheet of thickness 36 folded to fit intoa secondary preform 37 made circular in cross-section by the use ofsoluble glass 38. After elongation by drawing, cutting to separate fromthe remaining preform and dissolving away the soluble glass 38 drawnfolded sheet material 35' is heated and shaped by unfolding at one end39 (see FIG. 4(b)) but fused together in rectangular form at the otherend 40. Thus a coupler is formed which has a low aspect ratio at one endat a comparatively high such ratio at the other end. The arrangement issuitable for coupling a wide flat strip waveguide to a square sectionoptical waveguide.

In order to manufacture a coupler for use between a circular opticalwaveguide and a wide flat strip waveguide, the preform shown in FIG.5(a) may be used. This preform is made by rolling up a sheet ofinsoluble glass to form a primary preform 41 with a layer of solubleglass 42 so that a secondary preform is made which has a cross-sectionwith a substantially circular periphery. Internally the secondarypreform has two spirals (as shown in FIG. 5(a)). one of the spiralsbeing made of the relatively insoluble glass and the other of solubleglass. After elongation the coupler of FIG. 5(b) can be made bydissolving the soluble glass 42, heating and unwinding the insolubleglass at one end 43 so that it is suitable for coupling to a wide flatstrip waveguide and consolidating and fusing together the spiral ofinsoluble glass at the other end 44 so that it is suitable for couplingto a circular cross-section optical fibre.

Three ways of making directional couplers are now described. In thefirst, in which a polarising-preserving coupler is made, a secondarypreform is made up as shown in FIG. 6(a) from two primary preforms, eachconsisting of a rectangular cross-section core 44 in cladding glass 46,both of these glasses being relatively insoluble. Additional solubleglass 47 is positioned between the primary preforms and the whole formsa circular cross-section secondary preform 48. After elongation bydrawing and cutting the material 47 is dissolved over a length 49 only,and the waveguides 44' (derived from the cores 44) with their cladding46 are heated, deformed and fixed so that they are adjacent to oneanother. As a result coupling is achieved where the waveguides 44' arenear enough for their electromagnetic fields to interact but there is nocoupling over portions 51 and 52 where the waveguides 44' are far enoughapart and separated by the glass 47 to prevent coupling occurring.

The second way of making directional couplers starts, for apolarising-preserving coupler, by using a preform as shown in FIG. 7(a)where cores 53 with cladding 55 are positioned adjacent to one anotherin the secondary preform which is given a circular cross-section by theaddition of soluble glass 56. After elongation by drawing, and cutting,the soluble glass 56 is dissolved away in regions 57 and 58 but not in acentral region 59. Also the waveguides 53' (derived from the cores 53)together with their cladding 55 are heated and separated at the ends ofthe regions 57 and 58 so that they are too far apart to provide couplingbetween the waveguides 53'. Hence coupling only occurs in the centralregion 59 and those parts of the regions 57 and 58 where the waveguidesare adjacent to one another. Coupling between the waveguides can beadjusted by varying the length of the section 59 or by fusing togetherthose parts of the portions 57 and 58 where the waveguides are adjacentto one another.

In a third method of making directional couplers, the two opticalwaveguides which are to be coupled to make a polarising-preservingcoupler can be made in ridge and groove structures, respectively, sothat the waveguides can be positively located adjacent to one another.In FIG. 8(a) a primary preform 60 includes a core 61 in a ridge ofcladding material 62 and another primary preform 63 contains a core 64adjacent to a groove 65 in cladding material. The preforms 60 and 63 areheated and separated by soluble glass 66. When the soluble glass hasbeen dissolved after the preform has been elongated and cut the ridge 62is located in the groove 65 and the two fused together over a region 67as indicated in FIG. 8(b), the length of the region 67 providing therequired degree of coupling. The preforms 60 and 63 are heated andseparated in regions 68 and 69 so that no coupling occurs between thewaveguides 61' and 64' derived from the cores 61 and 64, respectively.In a variation of this construction the cores 60 and 64 each with itscladding are made separately as primary preforms, one with alongitudinal ridge and one with a longitudinal groove, by a method ofthe invention by using soluble glass to make separate secondary preformsof circular cross-section (not shown). The secondary preforms are eachdrawn and etched to remove the soluble glass. The resulting fibres arefused together with the ridge fitted in the groove over a regionsufficient to give the required coupling but they are shaped so thatthey are separated elsewhere.

The secondary preform of FIG. 9(a) for a polarising coupler is similarto that of FIG. 7(a) except that the two rectangular cross-sectionoptical cores 54 are located adjacent to the top of the rectangle formedby the cladding material 55. The object of this positioning is to allowwaveguides 54' derived from the cores 54 to be coated with an absorptioncoating 71 shown in FIG. 9(b) illustrating the finished coupler. Lightof one polarisation in the waveguides 54' is absorbed by the coating 71so giving the polarising action. The coating 71 is applied after thewaveguides in the regions 57 and 58 have been heated and separated fromone another.

The invention may also be used to manufacture optical sensors based onthose described by M. D. DeGrandpre and L. W. Burgess in "Long PathFibre Optic Sensors for Evanescent Field Absorption Measurements", Anal.Chem., 1988, No. 60, pages 2852-2856. These sensors have an optical corewith the largest possible surface area in order to couple an externalfield of supplied light to the sensor efficiently. The large surfacearea is coated with or surrounded by light absorbent medium.Alternatively similar arrangements can be used for emitting light bycoating the surface area with fluorescent material. A preform for makinga sensor of this type is shown in FIG. 10(a) where the primary preformcomprises a "star shape" 73 formed of a relatively insoluble glasssurrounded by soluble glass 74 to give the required circularcross-section for the secondary preform. The glass 74 is dissolved fromthe secondary preform after drawing and cutting, and the radialextensions from the core of the star shape are bent over and fusedtogether at ends 75 and 76 of the sensing element as shown in FIG.10(b). A central region 77 is either coated with or immersed inabsorbent or fluorescent material (not shown). The ends 75 and 76 allowconventional multi-mode fibres to be used as signalling and feed-backconnections.

An example of the application of methods of the invention applied to themanufacture of endoscopes is now described. In some circumstances it isan advantage if the image produced at one end of the endoscope isenlarged compared with that other end which receives light from theobject being observed.

The manufacture of such an endoscope is now described. Various bundlesof optical fibres can be used and an example is now mentioned.Individual fibres for bundling together to form an endoscope are shownin FIGS. 11(a) to 11(d). In these figures the optical core material isdesignated as 80 and its cladding as 81. In FIGS. 11(b) and 11(d) thecladding is surrounded by soluble glass 82. Groups of large numbers ofthese fibres with or without the soluble glass 82 are grouped togetheras shown at 83 in FIG. 12(a), with FIG. 12(b) showing an enlargedportion of the array 83 containing fibres of the type shown in FIG.11(b). The bundle 83 which forms the primary preform is surrounded bysoluble glass 84 to give the required circular cross-section and thesecondary preform so produced is drawn by the process described inconnection with FIG. 1.

However, the drawn preform is left without cutting or so cut that anenlarged end remains which has not been drawn, or is only partiallydrawn. The acid soluble material is then removed except at ends 93 and94 (see FIG. 13). The end 94 has not been through the heating zone shownin FIG. 1, and as a result it is of enlarged cross-section so that theimage appearing on the ends of the fibre is large in comparison with theobject observed at the other end, and since the fibres between theseends are not joined together the bundle is flexible between the ends andcan be used in an endoscope.

Another feature of the invention is now described in which a secondsoluble glass having a different solubility from the first is used.

The production of the couplers shown in FIG. 4(b) can be made easier bythe employment of this feature. A secondary preform for this purpose isshown in FIG. 14(a) where a folded sheet of insoluble glass 35' islocated between glass 34 which is less soluble than the glass 38. Theglass 38 makes up the circular section secondary preform required fordrawing and when this glass has been dissolved from the drawn preform,the product shown in FIG. 14(b) results. This figure shows the glass 35'held by gripping tools 96 which, after heating the product, are used topull out one end to give the end 39 (see FIG. 4(b)) a wide elongatedcross-section, and to compress the folds of the glass 35' at the otherend to give the rectangular end 40. After this shaping process the glass34 is removed in a second dissolving process.

In addition to the ways for producing a coupling element for use, forexample, between a diode array and an optical fibre described inconnection with FIGS. 3(a) and 3(b). a method is now described whichuses the second soluble glass. The secondary preform used (see FIG.15(a)) comprises rectangular cores coated with cladding glass 97 spacers(some of which are designated 98) of a less soluble glass, and moresoluble glass 90 making up the circular cross-section. After drawing andcutting the glass 90 is dissolved leaving the elongated bundle ofrectangular fibres and spacers 100 shown in FIG. 15(b). This bundle isthen connected to a diode array 101 located on a base plate 102. In thearray 101 the diodes (not shown) are spaced by an amount determined bythe construction of the array and therefore the spacers 98 are initiallydesigned to accommodate this spacing, taking into account the reductionin size due to drawing. After connection to the diode array the lesssoluble glass spacers 98 are dissolved in the region 103 (FIG. 15(c))and the resulting free optical fibres are gathered together and fused toform an end 104 which has a suitable cross-section for coupling to anoptical fibre to be attached at the end 104.

As has been mentioned the invention may also be used for non-opticalelements such as micromechanical elements, for example the componentshown in FIGS. 16(a) and 16(b) which has several channels 112 withconnections 113 defined in a particular way for the separation ofradio-isotopes. A primary preform 111 has a circular outer periphery incross-section and an internal shape defined by internal peripheries.This internal shape is to be preserved after the preform has been drawn.For this purpose the channels 112 are filled with a first more solubleglass while the internal connections 113 are filled with a secondrelatively less soluble glass. The component to be constructed is shownin FIG. 16(b) and has one end 114 which has not been necked down by thedrawing process and the other end 115 which has been drawn. For thiscomponent the internal connections between the channels are to remain atthe end 114 but removed at the other end 115. Thus the first leachingprocess dissolves away the glass 112 throughout the whole component buta second leaching process which dissolves away the glass 113 is onlycarried out in the necked down portion for some way along from the end115.

Embodiments of the invention have been specifically described above butit can, of course, be put into practice in many other ways.

I claim:
 1. A method of manufacturing objects from material which can bedrawn plastically, comprising the steps of:forming a secondary preformfrom at least one primary preform formed in a first glass by combining asecond glass with the primary preform, the first and second glasses bothbeing deformable plastically and one of the said glasses being removablefrom the other glass, reducing the viscosity of a zone of the secondarypreform while plastically drawing out the secondary preform to give anelongated product with a reduced cross-section, the secondary preformhaving a cross-sectional boundary whose shape, as defined by relativedimensions, is preserved during drawing by the presence of the first andsecond glasses, removing the one glass from at least part of theelongated product, said removing step including etching the one glass,and plastically deforming the material of the elongated product, afterthe one glass has been at least partially removed, to reshape theproduct permanently.
 2. A method according to claim 1 wherein the oneglass is the second glass and the primary preform or preforms have across-sectional outer or inner periphery which is preserved duringdrawing by the presence of the second glass.
 3. A method according toclaim 2 wherein:the first glass includes third and fourth glasses, thethird glass being removable from the fourth glass, and at leastpartially removing the third glass from the fourth glass from the oneglass has been at least partially removed from the first glass.
 4. Amethod according to claim 3 wherein after the second material has beenat least partially removed, the third glass facilitates manipulation ofthe first glass before it is removed.
 5. A method according to claim 2of manufacturing optical coupling elements wherein the secondary preformincludes a plurality of primary preforms having cross-sectional shapeswhich can be assembled to conform to first and second cross-sectionalshapes of respective optical devices which are to be coupled optically,andthe step of plastically deforming the product includes arrangingthose portions of the first glass remaining after the second glass hasbeen removed to assemble the cross-sectional shapes of the portions togive two ends to the product, the ends having cross-sectional shapeswhich conform with the first and second cross-sectional shapes,respectively.
 6. A method of manufacturing optical coupling elementsaccording to claim 2, for coupling a plurality of optical devices to asingle device wherein:the secondary preform includes a plurality ofprimary preforms each of which has a cross-sectional shape whichcorresponds to the cross-sectional shape of respective first opticaldevices, the cross-sectional shapes of the primary preforms beingcapable of assembly to conform with the cross-sectional shape of asecond optical device, and the step of plastically deforming the productincludes separating those portions of the first glass remaining afterthe second glass has been removed at one end to provide respectiveterminations for coupling the first optical devices, and assembling theportions at the other end to give a cross-sectional shape whichcorresponds to that of the second optical device.
 7. A method ofmanufacturing optical coupling elements according to claim 2, forcoupling a number of optical devices to a single such device wherein:thesecondary preform includes a plurality of primary preforms each of whichhas a cross-sectional shape which corresponds to the cross-sectionalshape of respective first optical devices, the cross-sectional shapes ofthe primary preforms being capable of assembly to conform with thecross-sectional shape of a second optical device, the step of removingthe second glass is partial, the second glass not being removed at oneend of the product whereby, after drawing, the ends of those portions ofthe first glass at the said one end are spaced by the second glass toconform with the spacing of the first optical devices, and the step ofplastically deforming the product includes assembling the portions atthe other end to give a cross-sectional shape which corresponds to thatof the second optical device.
 8. A method according to claim 2 ofmanufacturing optical coupling elements, wherein the primary preform hasa cross-sectional shape which can be rearranged to conform to first andsecond cross-sectional shapes of respective optical devices which are tobe coupled optically, andthe step of plastically deforming the productincludes shaping the first glass after the second glass has been removedto rearrange the cross-sectional shape of the first glass at the ends ofthe said product to give the said ends cross-sectional shapes whichcorrespond with the first and second cross-sectional shapes,respectively.
 9. A method according to claim 2 of making an opticaldirectional coupler wherein,the secondary preform includes two primarypreforms each comprising a core and cladding separated by secondaryglass, the cores of the primary preforms being rectangular incross-section and near the surface of the cladding where the primarypreforms are adjacent but the secondary glass being of such thicknessand composition after drawing that significant coupling between opticalwaves in the cores cannot occur, and the step of removing the secondglass includes removal at least at an intermediate portion of theproduct, and the step of plastically deforming the product includeslocating the drawn cores in close proximity in the intermediate portion.10. A method according to claim 2 of making an optical directionalcoupler wherein,the secondary preform includes two primary preforms eachcomprising a core and cladding, the cores of the primary preforms beingrectangular in cross-section and near the surface of the cladding wherethe primary preforms are adjacent to ensure that after drawing couplingoccurs between optical waves in the cores, and the step of removing thesecond glass includes removal at ends of the product, the step ofplastically deforming the product includes separating the cores, and themethod includes cladding at the said ends to prevent significantcoupling between optical waves in the cores.
 11. A method according toclaim 9 or 10 wherein the cores are also near the surface of thecladding where the primary preforms correspond to regions, of theproduct adjacent to positions in which coupling occurs between light inthe cores, and in which the second glass is removed, andthe methodincluding coating the said regions with a material which interacts withlight in the cores after the second glass has been removed.
 12. A methodof manufacturing an optical directional coupler,comprising:manufacturing first and second optical waveguides accordingto claim 2, each of which comprises a core surrounded by cladding with aregion in which the core is near an adjacent surface of the cladding,the region in the first optical waveguide comprising a longitudinalridge along the adjacent surface of the first optical waveguide, theregion in the second optical waveguide being adjacent to a longitudinalgroove in the adjacent surface of the second optical waveguide, and theridge and groove and the adjacent surfaces matching in that the ridgejust fits into the groove with the cores adjacent to one another whenthe adjacent surfaces touch, and the step of plastically deforming theproduct includes fixing the first and second optical waveguides togetherwith the ridge fitted into the groove along an intermediate region ofeach first and second optical waveguide but with the ends of thewaveguides spaced apart.
 13. A method of manufacturing objects frommaterial which can be drawn plastically, comprising the steps of:forminga secondary preform from at least one primary preform formed in a firstglass by combining a second glass with the primary preform, the firstand second glasses being plastically deformable and one of the glass canbe removed from the other glass, reducing the viscosity of a zone of thesecondary preform while plastically drawing out the secondary preform togive an elongated product with a reduced cross-section, the secondarypreform having a cross-sectional boundary whose shape, as defined byrelative dimensions, is preserved during drawing by the presence of thefirst and second glasses, and removing the one glass from at least partof the said elongated product via etching, wherein the first glassincludes third and fourth glasses, the third glass being removable fromthe fourth glass, and wherein the method includes at least partiallyremoving the third glass from the fourth glass after the one glass hasbeen at least partially removed from the first glass.
 14. A methodaccording to claim 13 wherein the said one glass is the second glass andthe primary preform or preforms have a cross-sectional outer or innerperiphery which is preserved during drawing by the presence of thesecond glass.
 15. A method according to claim 14 for manufacturing anoptical coupler for coupling spaced optical devices to a single opticaldevice, includingforming the primary preform from optical fibres, eachcomprising a core and cladding, by spacing the fibres with the thirdglass to conform after drawing to the spacing of the spaced opticaldevices, wherein the step of removing the third glass comprises removingthe third glass from the drawn product except at one end, and the methodincludes forming the fibres at the other end of the product into across-section which is suitable for coupling to the single opticaldevice.
 16. A method according to claim 14 including forming the primarypreform with the first glass having a cross-section with a circularouter periphery and a non-circular inner periphery, andforming thesecondary preform by filling the region bounded by the inner peripherywith the second glass.
 17. A method according to claim 16,includingforming the primary preform so that the inner peripherycomprises respective regions of the third and fourth glasses.
 18. Amethod according to claim 17 for manufacturing a device for separatingradio-isotopes, including:forming the primary preform with the third andfourth glass defining a plurality of inner peripheries and the thirdglass filling a connection which joins a plurality of channels, eachhaving walls formed by respective ones of the inner peripheries, andwherein the product includes a portion at one end which has not beendrawn, and the third glass is only removed from that portion of theproduct which has been drawn, thereby connecting the channels in thatportion.
 19. A method of manufacturing objects from material which canbe drawn plastically, comprising the steps of:forming at least oneprimary preform in a first glass by bundling together a plurality ofoptical fibres, each of which has a core and cladding, with the fibresspaced in cross-section according to a predetermined requirement,forming a secondary preform from said at least one primary preform bycombining a second glass with the primary preform, the first and secondmaterials being plastically deformable and one of the glasses beingremovable from the other glass, reducing the viscosity of a zone of thesecondary preform while plastically drawing out the secondary preform togive an elongated product with a reduced cross-section, the secondarypreform having a cross-sectional boundary whose shape, as defined byrelative dimensions, is preserved during drawing by the presence of thefirst and second materials, and removing the one glass from theelongated product, except at ends of the elongated product, using aremoval agent, and wherein the product includes a portion at one endwhich has not been drawn, or has been only partially drawn.
 20. A methodaccording to claim 19 further comprisingcoating each fibre with a layerof the second material before bundling and fixing the coated fibrestogether to form the primary preform.
 21. A method according to claim 1,13 or 19 including coating the elongated product after the, at leastpartial, removal of the second glass with an optical material.
 22. Anobject manufactured according to a method of claim 1, 13 or
 19. 23. Amethod according to claim 1, 13 or 19 further including a removal agentthat is a liquid or gas in which the second glass dissolves, physicallyor chemically, more readily than the first glass.
 24. A method accordingto claim 1, 13 or 19 wherein the addition of the second glass to theprimary preform gives the secondary preform an outer periphery which iscircular in cross-section, or square in cross-section with a diagonaldimension of the square which is large compared with the largestcross-sectional dimension of the primary preform.
 25. A method accordingto claim 1, 13 or 19 wherein in order to reduce viscosity in the zone,the secondary preform is heated at the zone.